As wireless communications technologies develop, higher requirements are imposed on a system throughput and a system rate, and a MIMO (Multiple-Input Multiple-Output) technology emerges. In the MIMO technology, a spatial feature can be fully used to increase a system capacity without increasing transmit power and bandwidth. In addition, the MIMO technology plays an important role in increasing a peak rate and improving reliability of data transmission, expanding coverage, suppressing interference, increasing the system capacity and the system throughput. As requirements on a rate and spectral efficiency constantly increase, enhancement and optimization of the MIMO technology are always an important direction for LTE system evolution.
A 1D antenna is used in a conventional 2D MIMO technology. To be specific, the antenna is placed in only a horizontal direction. As shown in FIG. 1A and FIG. 1B, a fixed downtilt is used for all terminals, and a beam direction can be adjusted in only the horizontal direction. As technologies develop, a 2D antenna is introduced, that is, a 3D MIMO technology. As shown in FIG. 1C, FIG. 1D, and FIG. 1E, a beam direction of the 2D antenna can be adjusted in both the horizontal direction and a vertical direction, and beam direction adjustment is freer. In the 3D MIMO technology, an expression form of W1 is shown in Formula 1:
                              W          1                =                  [                                                                                                                X                      ~                                        1                                    ⊗                                                            X                      ~                                        2                                                                              0                                                                    0                                                                                                        X                      ~                                        1                                    ⊗                                                            X                      ~                                        2                                                                                ]                                    (                  Formula          ⁢                                          ⁢          1                )            
where {tilde over (X)}1 and {tilde over (X)}2 are vectors in different dimensions or vector combinations. In this case, when feeding back a W1-related PMI to a base station, a terminal needs to feed back PMIs (Precoding Matrix Indicators) in two dimensions. However, currently, there is no CSI measurement and feedback method specific to the 3D MIMO technology.